KR20190080815A - Cathode material with stable surface for secondary batteries and method for producing the same - Google Patents

Abstract

A positive electrode material for a secondary battery according to an embodiment comprises: a positive electrode active material particle; and a surface coating layer disposed on a surface of the positive electrode active material particle and containing a material represented by chemical formula 2, which is ACaPO_4. In the chemical formula 2, A is an alkali metal. The alkali metal can be lithium or sodium.

Description

[0001] The present invention relates to a positive electrode material for a secondary battery having a stable surface and a method for producing the same,

The present invention relates to a secondary battery, and more particularly, to an alkali metal secondary battery.

The secondary battery refers to a battery that can be charged repeatedly as well as discharged. In a typical lithium secondary battery of the secondary battery, lithium ions contained in the positive electrode active material are transferred to the negative electrode through the electrolyte and then inserted (filled) into the layered structure of the negative electrode active material. Then, lithium ions, which have been inserted into the layered structure of the negative electrode active material, It works through the principle of returning to the anode (discharging). Such a lithium secondary battery is currently commercialized and is being used as a small power source for a cellular phone, a notebook computer, and the like, and it is expected that the lithium secondary battery will also be usable as a large power source such as a hybrid car, and the demand is expected to increase.

However, the composite metal oxide, which is mainly used as a cathode active material in a lithium secondary battery, is inferior in reliability due to deterioration due to reaction with an electrolyte.

The cathode materials of the secondary batteries developed so far are still not excellent in stability, and it is known that the batteries using the cathode materials need to improve the discharge capacity retention rate and stability.

Accordingly, it is an object of the present invention to provide an active material for a secondary battery having improved stability and a secondary battery including the same.

According to an aspect of the present invention, there is provided a cathode material for a secondary battery. The cathode material for a secondary battery includes a cathode active material particle; And a surface coating layer disposed on the surface of the cathode active material particle and containing a substance represented by the following formula (2).

(2)

ACaPO ₄

In Formula 2, A is an alkali metal. The alkali metal may be lithium or sodium.

The cathode active material may be represented by the following general formula (1).

[Chemical Formula 1]

A _x [Mn _{1 -yz} M ¹ _y M ² _z ] O _2-α Z _α

In the above formula (1), A is an alkali metal, x is 0.5 to 1, M ¹ and M ² are Sc, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Zr, Z is N, O, F, or N; y is 0 to 0.4; z is 0 to 0.4; and Z is N, N, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Al, Ga, Or S, and alpha is 0 to 0.1.

The surface coating layer may have a thickness of 1 to 20 nm.

According to another aspect of the present invention, there is provided a cathode material for a sodium secondary battery. The cathode material for a sodium secondary battery comprises a cathode active material particle; And a surface coating layer positioned on the surface of the cathode active material particle and exhibiting at least one of Na ₂ P ⁺ peak, Ca ₂ PO ⁺ peak, and NaCa ₂ PO ⁺ peak in a ToF-SIMS (Time of Flight Secondary Ion Mass Spectrometry) . The surface coating layer may be a layer containing a NaCaPO _4.

According to an aspect of the present invention, there is provided a method of manufacturing a cathode material for a secondary battery. First, phosphoric acid, a calcium salt, a cathode active material particle, and a solvent are mixed. The solvent is evaporated to obtain a powder. The powder is heat-treated to form a surface coating layer on the surface of the cathode active material particle.

The phosphate may be a H ₃ PO _4. The calcium salt may be calcium nitrate, calcium chloride, calcium acetate, calcium sulfide, or a combination of two or more of these. The solvent may be ethanol, acetone, or a mixture thereof. The solvent may be anhydrous ethanol. The heat treatment may be performed at a temperature of about 300 to 900 degrees in an oxygen atmosphere.

The surface coating layer may contain a substance represented by the following formula (2).

(2)

ACaPO ₄

In Formula 2, A is an alkali metal.

The surface coating layer may be a layer formed through the following Reaction Scheme 1 and / or Reaction Scheme 2.

[Reaction Scheme 1]

AOH + H ₃ PO ₄ + Ca (NO ₃ ) ₂ → ACaPO ₄ + 2HNO ₃ + H ₂ O

[Reaction Scheme 2]

A ₂ CO ₃ + 2H ₃ PO ₄ + ₂ Ca (NO ₃ ) ₂ → ₂ ACaPO ₄ + ₄ HNO ₃ + H ₂ CO ₃

In the above reaction scheme 1 or 2, A may be an alkali metal, and AOH and A ₂ CO ₃ may be alkali metal compounds remaining on the surface of the cathode active material particle.

The surface coating layer may exhibit at least one of a Na ₂ P ⁺ peak, a Ca ₂ PO ⁺ peak, and a NaCa ₂ PO ⁺ peak in a ToF-SIMS (Time of Flight Secondary Ion Mass Spectrometry) analysis.

According to the present invention, a cathode material having improved stability such as thermal stability can be obtained through surface treatment, and the battery using the cathode material can improve the discharge capacity retention rate and stability.

However, the effects of the present invention are not limited to the effects mentioned above, and other effects not mentioned can be clearly understood by those skilled in the art from the following description.

1 is a schematic view showing a secondary battery according to an embodiment of the present invention.
2A and 2B show XRD graphs of sodium transition metal oxide (before surface treatment) and surface treated sodium transition metal oxide (after surface treatment) according to the cathode active material comparison, respectively, according to the cathode active material preparation example.
FIG. 3 is a graph showing the results of SEM (scanning electron microscope), TEM (surface pre-treatment, bare) and SEM (transmission electron microscope), and selected area electron diffraction (SAED) photographs.
4 shows an SEM mapping of the surface-treated sodium transition metal oxide (coated after surface treatment) according to the cathode active material production example.
5 is a graph showing the results of a ToF-SIMS (bare) analysis of the surfaces of sodium transition metal oxide (bare before surface treatment) and surface treated sodium transition metal oxide (coated after surface treatment) Time-of-Flight secondary ion mass spectrometry data.
6 is an XRD graph of the powder obtained through the experimental example.
Fig. 7 is a graph showing the results of TG (thermogravimetry) and DSC (barium titanate) measurements for the sodium transition metal oxide (bare before surface treatment) and the surface treated sodium transition metal oxide (coated after surface treatment) Differential scanning calorimetry. < / RTI >
8 is a graph showing the results of a comparison between the temperature of the sodium transition metal oxide (bare before surface treatment) and the surface treated sodium transition metal oxide (coated after surface treatment) obtained in the cathode active material preparation example (room temperature -600 ° C) Fig.
9 is a graph showing charge / discharge characteristics of the secondary batteries according to the battery production example (coating) and the battery comparison example (bare).
FIG. 10 is a graph showing the discharge characteristics and the discharge capacity according to the number of cycles of the secondary batteries according to the battery manufacturing example (coated) and the battery comparison example (bare).
FIG. 11 is a graph showing the discharge characteristics and the discharge capacity according to the rate of the reversed charge according to the battery production example (coated) and the battery comparison example (bare).

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments described herein but may be embodied in other forms.

1 is a schematic view showing a secondary battery according to an embodiment of the present invention.

In this embodiment, the secondary battery is an example of an alkaline ion battery, and may be a lithium secondary battery or a sodium secondary battery. However, the present invention is not limited thereto.

Referring to FIG. 1, the secondary battery includes a negative electrode active material layer 120, a positive electrode active material layer 140, and a separator 130 interposed therebetween. The electrolyte 160 may be disposed or filled between the anode active material layer 120 and the separator 130 and between the cathode active material layer 140 and the separator 130. The anode active material layer 120 may be disposed on the anode current collector 110 and the cathode active material layer 140 may be disposed on the cathode current collector 150. The anode active material layer 120 and the anode current collector 110 may be referred to as a cathode and the cathode active material layer 140 and the anode current collector 150 may be referred to as a cathode.

<Anode>

In order to obtain a positive electrode, a positive electrode active material can be prepared. The cathode active material may be cathode active material particles in the form of particles. Specifically, the cathode active material may be a lithium-transition metal oxide or a sodium-transition metal oxide.

The cathode active material may be represented by the following general formula (1).

[Chemical Formula 1]

A _x [Mn _{1 -yz} M ¹ _y M ² _z ] O _2-α Z _α

In Formula 1, A may be an alkali metal, specifically, lithium or sodium. x may be from 0.5 to 1. Specifically, when A is lithium, x may be 0.9 to 1, and when A is sodium, x may be 0.5 to 0.8. M ¹ and M ² are each independently selected from Sc, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Ga, In, Sn, or Bi. Specifically, M ¹ and M ² may be Co or Ni, regardless of each other. y may range from 0 to 0.4. and z may range from 0 to 0.4. Z may be N, O, F, or S, and a may be from 0 to 0.1.

The cathode active material represented by Formula 1 may be a layered compound in which an alkali metal layer, specifically, a lithium layer or a sodium layer and a transition metal oxide layer are alternately stacked. The sodium transition metal oxide may be a sodium transition metal oxide having a P2 structure, and may be, for example, Na _0.7 [Ni _0.3 Mn _0.7 ] O ₂ .

The positive electrode active material particles are obtained by reacting the positive electrode active material powder in a treatment solution in which phosphoric acid and calcium salt are dissolved in a solvent and reacting and evaporating the solvent to obtain a powder and then heat treating the powder to form a surface coating layer formed on the particle surface have.

The phosphate may be a H ₃ PO _4. The calcium salt may be calcium nitrate, calcium chloride, calcium acetate, calcium sulphide, or a combination of two or more thereof. The calcium salt may be an acidic raw material capable of weakly acidifying the treatment solution so as to exhibit weak acidity. The solvent may be a volatile solvent, for example, an alcohol such as ethanol, acetone, or a mixture thereof. Specifically, the solvent may be an anhydrous solvent, specifically, anhydrous ethanol to minimize the exposure of the cathode active material to water. The evaporation of the solvent can be carried out at a temperature of 60 to 100 degrees. The heat treatment may be performed in an oxygen atmosphere at a temperature of about 300 to 900 degrees, specifically about 500 to 900 degrees, more specifically about 600 to 800 degrees, further about 650 to 750 degrees, for example about 680 to 730 degrees, 5 hours. &Lt; / RTI &gt;

In one example, the surface coating layer may contain a substance represented by the following formula (2).

(2)

ACaPO ₄

In Formula 2, A may be an alkali metal, specifically, lithium or sodium.

The material represented by Formula 2 may be formed through the following Reaction Scheme 1 and / or Reaction Scheme 2. However, the present invention is not limited thereto.

[Reaction Scheme 1]

AOH + H ₃ PO ₄ + Ca (NO ₃ ) ₂ → ACaPO ₄ + 2HNO ₃ + H ₂ O

[Reaction Scheme 2]

A ₂ CO ₃ + 2H ₃ PO ₄ + ₂ Ca (NO ₃ ) ₂ → ₂ ACaPO ₄ + ₄ HNO ₃ + H ₂ CO ₃

In the above Reaction Scheme 1 or 2, A may be an alkali metal, specifically lithium or sodium.

In the above reaction schemes, the alkali metal hydroxide (ex. AOH) or the alkali metal carbonate (ex. A ₂ CO ₃ ) may be an alkali metal compound remained on the cathode active material, specifically, a lithium compound or a sodium compound. Further, on the surface of the cathode active material, which is an alkali metal-transition metal oxide, there may exist an alkali metal compound which does not form an oxide with the transition metal. Such a residual alkali metal compound may react with a specific substance in the electrolyte in the secondary battery. And the reactants can be accumulated on the surface of the cathode active material. These reactants may interfere with the migration of alkali metal ions. As an example, the residual lithium compound may react with HF or the like in the electrolyte to form LiF, and the residual sodium compound may react with HF or the like in the electrolyte to generate NaF.

However, when the cathode active material is mixed with phosphoric acid and calcium salt in the solvent and then heat-treated as described above, a surface coating layer as shown in Formula 2 may be formed on the surface of the cathode active material. The surface coating layer may have a thickness of 1 to 20 nm, specifically 1 to 10 nm, for example 2 to 9 nm. This surface coating layer can serve to protect the cathode active material without interfering with the movement of the alkali metal ion. In addition, as the residual alkali metal compound is consumed in the process of forming the surface coating layer, deterioration of the cathode active material due to side reactions with the electrolyte can be prevented, and the formed surface coating layer can prevent deterioration of the cathode active material Lt; / RTI &gt; The surface coating layer may be formed on at least some of the surfaces of the cathode active material particles, specifically on the entire surface. The surface coating layer may further contain a small amount of the alkali metal compound, specifically, a lithium compound or a sodium compound, in addition to the substance represented by the formula (2).

In another example, the surface coating layer may exhibit at least one of a Na ₂ P ⁺ peak, a Ca ₂ PO ⁺ peak, and a NaCa ₂ PO ⁺ peak in a ToF-SIMS (Time of Flight Secondary Ion Mass Spectrometry) analysis.

Thereafter, the cathode active material coated with the surface coating layer, the conductive material, and the binder may be mixed to obtain a cathode material. At this time, the conductive material may be a carbon material such as natural graphite, artificial graphite, coke, carbon black, carbon nanotube, or graphene. The binder may be a thermoplastic resin such as polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene, vinylidene fluoride copolymer, fluorine resin such as propylene hexafluoride, and / or polyolefin resin such as polyethylene or polypropylene . The positive electrode material can be coated on the positive electrode collector to form the positive electrode. The positive electrode current collector may be a conductive material such as Al, Ni, or stainless steel. The positive electrode material may be applied on the positive electrode collector by a method such as a pressing method using a paste or an organic solvent, applying the paste on a current collector, and pressing and fixing the paste. Examples of the organic solvent include amine-based solvents such as N, N-dimethylaminopropylamine and diethyltriamine; Ethers such as ethylene oxide and tetrahydrofuran; Ketone type such as methyl ethyl ketone; Esters such as methyl acetate; Aprotic polar solvent such as dimethylacetamide, N-methyl-2-pyrrolidone and the like. The paste may be applied on the positive electrode current collector using, for example, a gravure coating method, a slit die coating method, a knife coating method, or a spray coating method.

<Cathode>

The negative electrode active material may be selected from the group consisting of metals, metal alloys, metal oxides, metal fluorides, metal sulfides, and natural graphite, artificial graphite, coke, and the like, which can dissolve alkali metal ions, that is, lithium ions or sodium ions, Carbon black, carbon nanotubes, or carbon material such as graphene.

A negative electrode material can be obtained by mixing a negative electrode active material, a conductive material, and a binder. At this time, the conductive material may be a carbon material such as natural graphite, artificial graphite, coke, carbon black, carbon nanotube, and graphene. The binder may be a thermoplastic resin such as polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene, vinylidene fluoride copolymer, fluorine resin such as propylene hexafluoride, and / or polyolefin resin such as polyethylene or polypropylene .

The anode material can be applied on the anode current collector to form the anode. The positive electrode current collector may be a conductive material such as Al, Ni, or stainless steel. The negative electrode material may be applied to the positive electrode current collector by press molding, or by making a paste using an organic solvent or the like, applying the paste to a current collector, and pressing and fixing the paste. Examples of the organic solvent include amine-based solvents such as N, N-dimethylaminopropylamine and diethyltriamine; Ethers such as ethylene oxide and tetrahydrofuran; Ketone type such as methyl ethyl ketone; Esters such as methyl acetate; Aprotic polar solvent such as dimethylacetamide, N-methyl-2-pyrrolidone and the like. The paste may be applied on the negative electrode current collector by, for example, a gravure coating method, a slit die coating method, a knife coating method, or a spray coating method.

<Electrolyte>

The electrolyte may be, for example, an alkali metal salt, a lithium salt or a sodium salt. The lithium salt is selected from the group consisting of lithium perchlorate (LiClO ₄ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium trifluoromethane sulfonate (LiCF ₃ SO ₃ ), lithium hexafluoroacetate _6), lithium trifluoro methane sulfonyl imide _{(Li (CF 3 SO 2)} 2 N), or may be those of two or more kinds of mixtures, the sodium salt is NaClO _4, NaPF _6, NaAsF _6, NaSbF _6, NaBF ₄ , NaCF ₃ SO ₃ , NaN (SO ₂ CF ₃ ) ₂ , a lower aliphatic carboxylic acid sodium salt, NaAlCl _4, or a mixture of two or more thereof. Of these, an electrolyte containing fluorine can be used. Further, the electrolyte may be dissolved in an organic solvent and used as a non-aqueous electrolyte. Examples of the organic solvent include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, isopropyl methyl carbonate, vinylene carbonate, 4- Carbonates such as trifluoromethyl-1,3-dioxolan-2-one and 1,2-di (methoxycarbonyloxy) ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropylmethylether, 2,2,3,3-tetrafluoropropyldifluoromethylether, tetrahydrofuran, 2-methyltetrahydro Ethers such as furan; Esters such as methyl formate, methyl acetate and? -Butyrolactone; Nitriles such as acetonitrile and butyronitrile; Amides such as N, N-dimethylformamide and N, N-dimethylacetamide; Carbamates such as 3-methyl-2-oxazolidone; Sulfur-containing compounds such as sulfolane, dimethylsulfoxide, and 1,3-propanesultone; Or an organic solvent in which a fluorine-substituted group is further introduced may be used.

Alternatively, a solid electrolyte may be used. The solid electrolyte may be an organic solid electrolyte such as a polymer compound of a polyethylene oxide system, a polymer compound containing at least one or more of a polyorganosiloxane chain or a polyoxyalkylene chain. A so-called gel type electrolyte in which a non-aqueous electrolyte is supported on the polymer compound may also be used. On the other hand, an inorganic solid electrolyte may be used. In some cases, the safety of the sodium secondary battery can be enhanced by using these solid electrolytes. Further, the solid electrolyte may serve as a separator to be described later, in which case a separator may not be required.

<Separator>

A separator may be disposed between the anode and the cathode. Such a separator may be a material having a form such as a porous film made of a material such as a polyolefin resin such as polyethylene or polypropylene, a fluororesin or a nitrogen-containing aromatic polymer, a nonwoven fabric or a woven fabric. The thickness of the separator is preferably as thin as long as the mechanical strength is maintained in that the volume energy density of the battery is high and the internal resistance is small. The thickness of the separator may generally be about 5 to 200 mu m, and more specifically, 5 to 40 mu m.

&Lt; Method of producing alkaline metal secondary battery >

A secondary battery can be manufactured by stacking an anode, a separator, and a cathode in this order to form an electrode group, if necessary, the electrode group is rolled up and stored in a battery can, and the electrode group is impregnated with a non-aqueous electrolyte. Alternatively, the positive electrode, the solid electrolyte, and the negative electrode may be laminated to form an electrode group, and if necessary, the electrode group may be rolled up and stored in a battery can to manufacture a secondary battery.

Hereinafter, exemplary embodiments of the present invention will be described in order to facilitate understanding of the present invention. It should be understood, however, that the following examples are intended to aid in the understanding of the present invention and are not intended to limit the scope of the present invention.

Comparative Example of Cathode Active Material: Sodium transition metal oxide

Na ₂ CO ₃ , Mn ₂ O ₃ , and NiO were quantified according to the molar ratio, and then the sodium transition metal oxide was obtained using the solid phase method. Specifically, the quantified materials were heat-treated at a temperature of 1000 degrees for 12 hours while mixing and stirring, and Na _0.7 [Ni _0.3 Mn _0.7 ] O ₂ powder was obtained through rapid cooling.

Preparation of cathode active material: Surface-treated sodium transition metal oxide

Calcium nitrate and phosphoric acid were added to ethanol, and the solution was sufficiently stirred to obtain a solution. Na _0.7 [Ni _0.3 Mn _0.7 ] O ₂ powder prepared in the above cathode active material comparative example was put into this solution and sufficiently stirred. Thereafter, the solvent was evaporated at 80 ° C to obtain a powder, and the powder was heat-treated at 700 ° C for 2 hours.

2A and 2B show XRD graphs of sodium transition metal oxide (before surface treatment) and surface treated sodium transition metal oxide (after surface treatment) according to the cathode active material comparison, respectively, according to the cathode active material preparation example.

2A and 2B, it can be seen that the peaks before and after the surface treatment of the sodium transition metal oxide are identifiable, indicating that there is no structural difference in the sodium transition metal oxide itself before and after the surface treatment. However, the layer formed by the surface treatment is a very thin layer having a thickness of about 10 nm or less as shown below, and it is judged that this layer does not appear in this XRD graph.

FIG. 3 is a graph showing the results of SEM (scanning electron microscope), TEM (surface pre-treatment, bare) and SEM (transmission electron microscope), and selected area electron diffraction (SAED) photographs. 4 shows an SEM mapping of the surface-treated sodium transition metal oxide (coated after surface treatment) according to the cathode active material production example.

3 and 4, it can be seen that a surface coating layer of about 10 nm or less, specifically about 2 to 9 nm, is coated on the surface of the sodium transition metal oxide, and the surface coating layer is composed of Na, Ca, P and O as elements.

5 is a graph showing the results of a ToF-SIMS (bare) analysis of the surfaces of sodium transition metal oxide (bare before surface treatment) and surface treated sodium transition metal oxide (coated after surface treatment) Time-of-Flight secondary ion mass spectrometry data.

Referring to Figure 5, NaOH ⁺ ₂ peak, NaC ₂ ⁺ peak, and a peak intensity of NaCO ⁺ It can be seen that the reduced after treatment compared to the surface-treated surface (bare) (coated). The NaOH ₂ ⁺ peak is related to NaOH on the sodium transition metal oxide surface and the NaC ₂ ⁺ peak and the NaCO ⁺ peak are related to Na ₂ CO ₃ on the sodium transition metal oxide surface, And Na ₂ CO _3, that is, the amount of residual sodium.

On the other hand, the intensities of the Na ₂ P ⁺ peak, Ca ₂ PO ⁺ peak, and NaCa ₂ PO ⁺ peak are newly detected or increased in strength after the surface treatment with respect to the bare surface. These Na ₂ P ⁺ peaks, Ca ₂ PO ⁺ peaks, and NaCa ₂ PO ⁺ peaks can be assumed to be detected from the surface coating layer coated on the sodium transition metal oxide surface obtained from the production example.

The following Table 1 shows the results of a comparison between the sodium transition metal oxide (bare before surface treatment) and the residual sodium detected on the surface of the surface-treated sodium transition metal oxide (coated after surface treatment) obtained in the cathode active material preparation example Amount.

[Table 1]

Referring to Table 1, it can be seen that the amount of residual sodium compounds remaining on the surface of the sodium transition metal oxide after the surface treatment, that is, the amount of NaOH and Na ₂ CO ₃ , is greatly reduced.

Experimental Example

0.4 M Na ₂ CO ₃ , 0.1 M phosphoric acid and 0.1 M calcium nitrate were added to ethanol and stirred. Then, the solvent was evaporated at 80 ^° C and the powder was heat-treated at 700 ^° C.

6 is an XRD graph of the powder obtained through the experimental example.

Referring to FIG. 6, it was confirmed that the material obtained through the experimental example is in agreement with the XRD data of JCPDS (Joint Committee on Powder Diffraction Standards) of NaCaPO ₄ .

Referring to Table 1, it can be seen that there are many Na ₂ CO _{3 on the} surface of the sodium transition metal oxide (bare). The Na ₂ CO ₃ used in this experiment was 0.4 M based on the titration of the sodium transition metal oxide (bare), which reacted very vigorously with the phosphorus acid and calcium nitrate coating materials in ethanol. From this, it can be deduced that Na ₂ CO ₃ , the residual sodium compound on the surface of the sodium transition metal oxide (bare), reacts with phosphoric acid and calcium nitrate to form a surface coating layer of NaCaPO _{4 on} the surface of the sodium transition metal oxide have.

Fig. 7 is a graph showing the results of TG (thermogravimetry) and DSC (barium titanate) measurements for the sodium transition metal oxide (bare before surface treatment) and the surface treated sodium transition metal oxide (coated after surface treatment) Differential scanning calorimetry. &lt; / RTI &gt;

Referring to FIG. 7, it can be seen that the surface-treated sodium transition metal oxide has improved thermal stability.

8 is a graph showing the results of a comparison between the temperature of the sodium transition metal oxide (bare before surface treatment) and the surface treated sodium transition metal oxide (coated after surface treatment) obtained in the cathode active material preparation example (room temperature -600 ° C) Fig.

Referring to FIG. 8, it can be confirmed once again that the thermal stability is improved by the surface treatment because the XRD peaks are hardly changed even at a high temperature when the surface treatment is performed.

Battery Manufacturing Example

(1) anode

The slurry was prepared by stirring the surface-treated cathode active material, denka black (super P) and binder (PVdF) obtained in the preparation of the cathode active material in NMP at a ratio of 8: 1: 1, Cast and dried to form an anode.

(2) Battery

The obtained anode, glass filter separator, sodium metal cathode, and electrolyte (3% NaPF6 in PC + FEC) were prepared to prepare a half-cell.

Battery Comparison Example

A reversed cell was prepared in the same manner as in the battery comparison example except that the Na _0.7 [Ni _0.3 Mn _0.7 ] O ₂ powder obtained in the cathode active material comparative example was used as a cathode active material without surface treatment.

9 is a graph showing charge / discharge characteristics of the secondary batteries according to the battery production example (coating) and the battery comparison example (bare). At this time, the constant current charging was carried out at a charging rate of 15 mA / g up to 4.3 V, and the discharging was performed at a rate equal to the charging rate up to a constant current discharge of 2.5 V. Charging and discharging were carried out for one cycle.

Referring to FIG. 9, the difference in initial cell performance between the surface-treated positive electrode active material and the non-surface-treated positive electrode active material is not so large. However, the surface-treated positive electrode active material has a somewhat higher reversibility, showing reversibility of 83.73% appear.

FIG. 10 is a graph showing the discharge characteristics and the discharge capacity according to the number of cycles of the secondary batteries according to the battery manufacturing example (coated) and the battery comparison example (bare). At this time, constant current charging was performed at a rate of 75 mA / g up to 4.3 V, and discharge was performed up to 2.5 V at the same rate as the charging rate. Charging and discharging proceeded for 200 cycles.

Referring to FIG. 10, it can be seen that the surface-treated cathode active material shows a very good capacity retention ratio as compared with the surface active agent.

FIG. 11 is a graph showing the discharge characteristics and the discharge capacity according to the rate of the reversed charge according to the battery production example (coated) and the battery comparison example (bare). At this time, charging / discharging was evaluated at a current density between 0.1 C (15 mA / g) and 10 C (1500 mA / g) in a voltage range of 2.5-4.3V.

Referring to FIG. 11, it can be seen that the surface-treated cathode active material exhibits a very superior rate characteristic to the surface active cathode active material.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, This is possible.

Claims (15)

Cathode active material particles; And
And a surface coating layer positioned on the surface of the cathode active material particle and containing a substance represented by the following Formula 2:
(2)
ACaPO ₄
In Formula 2, A is an alkali metal. The method according to claim 1,
Wherein the alkali metal is lithium or sodium. The method according to claim 1,
Wherein the cathode active material is represented by the following Formula 1:
[Chemical Formula 1]
A _x [Mn _{1 -yz} M ¹ _y M ² _z ] O _2-α Z _α
In the above formula (1), A is an alkali metal, x is 0.5 to 1, M ¹ and M ² are Sc, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Zr, Z is N, O, F, or N; y is 0 to 0.4; z is 0 to 0.4; and Z is N, N, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Al, Ga, Or S, and alpha is 0 to 0.1. The method according to claim 1,
Wherein the surface coating layer has a thickness of 1 to 20 nm. Cathode active material particles; And
A surface coating layer positioned on the surface of the cathode active material particle and exhibiting at least one of Na ₂ P ⁺ peak, Ca ₂ PO ⁺ peak, and NaCa ₂ PO ⁺ peak in ToF-SIMS (Time of Flight Secondary Ion Mass Spectrometry) A cathode material for a sodium secondary battery. 6. The method of claim 5,
Sodium secondary battery positive electrode material layer to the surface coating layer containing NaCaPO _4. Mixing phosphoric acid, a calcium salt, a cathode active material particle, and a solvent;
Evaporating the solvent to obtain a powder; And
And heat treating the powder to form a surface coating layer on the surface of the cathode active material particle. 8. The method of claim 7,
Wherein the phosphoric acid is H ₃ PO ₄ . 8. The method of claim 7,
Wherein the calcium salt is calcium nitrate, calcium chloride, calcium acetate, calcium sulfide, or a combination of two or more thereof. 8. The method of claim 7,
Wherein the solvent is ethanol, acetone, or a mixture thereof. 11. The method of claim 10,
Wherein the solvent is anhydrous ethanol. 8. The method of claim 7,
Wherein the heat treatment is performed at a temperature of 300 to 900 degrees in an oxygen atmosphere. 8. The method of claim 7,
Wherein the surface coating layer contains a substance represented by the following formula (2): &lt; EMI ID =
(2)
ACaPO ₄
In Formula 2, A is an alkali metal. 14. The method of claim 13,
The surface coating layer
A method for producing a cathode material for a secondary battery, which is a layer formed through the following Reaction Scheme 1 and / or Reaction Scheme 2:
[Reaction Scheme 1]
AOH + H ₃ PO ₄ + Ca (NO ₃ ) ₂ → ACaPO ₄ + 2HNO ₃ + H ₂ O
[Reaction Scheme 2]
A ₂ CO ₃ + 2H ₃ PO ₄ + ₂ Ca (NO ₃ ) ₂ → ₂ ACaPO ₄ + ₄ HNO ₃ + H ₂ CO ₃
In the above Reaction Scheme 1 or 2, A is an alkali metal,
AOH and A ₂ CO ₃ are alkali metal compounds remaining on the surface of the positive electrode active material particles. 8. The method of claim 7,
Wherein the surface coating layer exhibits at least one of a Na ₂ P ⁺ peak, a Ca ₂ PO ⁺ peak, and a NaCa ₂ PO ⁺ peak in a ToF-SIMS (Time of Flight Secondary Ion Mass Spectrometry) analysis.

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